In recent years, a hood for a vehicle such as a car is required to assure collision safety for a pedestrian and the like. For example, to assure safety of the head of a pedestrian at the time of collision, it is internationally requested to set head injury criteria (HIC) to be equal to or less than a reference value.
The head injury criteria are obtained by a pedestrian head safety test. The pedestrian head safety test is carried out by discharging dummies (head impactors) imitating the heads of a child and an adult from a collision apparatus to a hood to be tested, measuring the impact of the head impactors, and obtaining head injury criteria.
FIG. 6 shows an example of the result of the pedestrian head safety test. The diagram shows the relation between acceleration of the head impactor when the head impactor collides with a hood (hereinbelow, simply called acceleration) and time. Head injury criteria are obtained by integrating a curve G indicated by the relation of the acceleration and time (for details, refer to an embodiment to be described later).
As shown in the diagram, when the head impactor collides with the hood, the primary peak (primary peak P1) of acceleration of the head impactor appears. Since the head injury criteria are obtained by integrating the curve G indicated by the relation between acceleration and time as described above, when the primary peak P1 is high, the head injury criteria are also high.
For example, there is a hood covering and protecting an engine room of a car, having a structure that an insulator for suppressing transmission of sound generated in the engine room to the outside, a duct connected to an intercooler, and the like are disposed. In such a structure, a hood for use in a vehicle such as an SUV is larger than that of a passenger car or a 1-box car. If the hood is made of the same material, the mass of the entire hood is large and there is tendency that rigidity also becomes higher.
When the mass of the entire hood becomes large and rigidity becomes high, the primary peak in the pedestrian head safety test becomes high, and a problem accordingly occurs such that the head injury criteria become high. Consequently, a large hood is particularly requested to lower the head injury criteria.
To lower the head injury criteria, the primary peak has to be lowered. Although the structure of a vehicle body panel for lowering a secondary peak after a primary peak is proposed (refer to Patent Document 1), the structure of a hood capable of effectively lower a primary peak with a simple structure is not proposed.
Patent document 1: JP 2003-226264 Unexamined Patent Publication (Kokai)